My guess is that when the wind blows, the siding flexes and bows. As it does so, air rushes in underneath it at the seam. When it returns back to normal shape, the air is blown back out the seam taking any stirred up dust with it. It sticks to any moisture on the siding, and when it dries it leaves this pattern.
For the record I do not know this is happening, I am just positing it as a theory.
To add to Marks theory, two of them are in the same row, and facing opposite directions. Another reason to lap your siding all the same direction. Does this side get any sun? and we are looking at a gable, right?
That explains the short piece in the middle. I have seen countless homes (including my own) put together quickly. It is standard practice to only install house wrap on the exterior wall, and not the upper gable. You may have nailed it with the lack of house wrap. If it should happen to blow off in transport, it sometimes doesn’t get replaced. (same with insulation as far as that goes)
The seams are subject to expansion and contraction. The movement is depositing material on the surface from what is behind the siding (probably a type of mold).
As for the other comments dealing with not using a house wrap, remember that sheathing paper was not required to be used under vinyl siding, on homes (manufactured or otherwise) before the 2006 IRC. (Table 703.4).
As the seams are subject to expansion and contraction. I would suspect that it’s material on the back side of the siding that’s being deposited on the face after the siding cools and contracts. My guess is it’s mold.
As for the comments concerning the use of house wrap, remember that sheathing paper was not required on homes (manufactured or conventional) prior the the 2006 IRC.
IMO its ghosting.
The siding seams and weep slot exhausting trapped atmospheric grim (fine particulate) behind the siding.
If the window flange is not sealed vertical shear can load atmospheric grim between the VBR or VDB on the sub straight and the siding.
The dirt or ghosting marks appear to be atmospheric grim that gets trapped inside the siding.
Wind from one direction loads the grim.
Wind shear or positive pressure inside the home shear loading from another direction allows the atmospheric grim to be released.
Like ghosting on can light fixtures in a ceiling without the can being sealed…
dirt in the attic is passed through the opening.
NOTE: Its area spastic?
Any other areas where this exists on the exterior envelope.?
hypothetical (suspect) Window flange not sealed.
OR positive pressure inside that home,. The VDR or VDB was not taped properly in that area…
Siding is not tightly nailed Mr. Webber. It hangs as opposed to being anchored .
The whole wall expands and contracts at the same time and at the same rate.
The reason is to reduce material rubbing.
It prevents convex siding expatiation also.
If it was organic growth it would likely have a colour.
It would also need a source of fuel/energy. Water or high concentrations of RH. relative humidity.
The inside wall would most probably show paint shading, discolouration, staining and more.
Happy you are getting involved in HI home inspections.
The observation and hypothetical anatomy and hypotheses of defects and deficiencies in structure, system, components and their materials.
Sorry guys if my description is a little unique.
Its my definition. Sorry if I offended anyone.
While not required specifically by Building Code (IRC), it has been required by the Manufacturer for many years…
[FONT=HelveticaLTStd-Bold][size=2]Water Resistive Barrier
[/size][/FONT][FONT=HelveticaLTStd-BoldObl]Vinyl siding has always been designed as an exterior cladding, not a water-resistive barrier.[/FONT]
**[FONT=HelveticaLTStd-BoldObl]*Vinyl siding is designed to allow the material underneath it to breathe; therefore, it is not a ******watertight covering. ***
[FONT=HelveticaLTStd-Roman]*Because of its design and application, it provides a supplemental rain screen *[/FONT][FONT=HelveticaLTStd-Roman]*that enhances the water-resistive barrier system by reducing the amount of water that reaches the *underlying water-resistive barrier.
[/FONT][FONT=HelveticaLTStd-Bold][/FONT][FONT=HelveticaLTStd-Bold]*What Is a Water-Resistive Barrier System? *
[/FONT][FONT=HelveticaLTStd-Roman]It is a system that includes water shedding materials[/FONT]
[FONT=HelveticaLTStd-Roman]*and water diversion materials. Water-resistive barrier systems commonly consist of a combination *[/FONT][FONT=HelveticaLTStd-Roman][size=2]*of exterior cladding, flashed wall openings and penetrations, water-resistive barrier material, and **sheathing. Effective water-resistive barrier systems will shed the water initially, control moisture flow **by capillary and diffusion action, and minimize absorption into the wall structure. The level of water *resistance required is determined by the applicable building code and structure.